Influence of the wettability on the residual fluid saturation for homogeneous and heterogeneous porous systems

The influence of wettability on the residual fluid saturation is analyzed for homogeneous and heterogeneous porous systems. Several simulations under different wettability, flow rate, and heterogeneity conditions were carried out using a two-component lattice-Boltzmann method. The fluid flow driving force and initial conditions were imposed using a specific methodology that allows a clear distinction between the results obtained for immiscible displacement when the porous medium is initially saturated with one fluid (called primary) and when two fluids are filling the porous spaces (called secondary). The results show that the primary sweeping process is more effective when the displaced fluid is non-wetting. We observe that the heterogeneity has an important role for the whole process since it disturbs the fluid interfaces inducing the flow in the longitudinal and transversal directions, improving considerably the effectiveness of the primary displacement when compared with ideally homogeneous cases. We noted that for oil contact angles, theta(o), higher than a critical value, no residual oil is found. In all homogeneous cases, the critical value is 120 degrees. The residual fluid increases proportionally to the capillary number for primary displacements, but it also depends on the system heterogeneity and wetting conditions. For secondary displacements in heterogeneous systems, the highest residual oil saturation is found for completely oil-wet conditions, with values ranging from 29% to 41% and tending to zero for all cases when theta(o) > 120 degrees. The initial water-oil distribution is found to be a determining factor in the amount of trapped oil after the waterflooding process.